An electro-optic channel waveguide is a three-dimensional optical waveguide formed as a channel in a structure made of a non-linear optical material for the purpose of transmitting selected modes of optical radiation of desired polarization. The index of refraction in an electro-optic channel waveguide is greater for at least one polarization in at least one direction than the index of refraction in portions of the polymer structure adjacent the channel waveguide.
Techniques for fabricating three-dimensional optical waveguides made of thin-film materials are described in U.S. Pat. 4,767,169 granted on Aug. 3, 1988, which is assigned to Hoechst-Celanese Corporation. Typically, a thin-film optical waveguide is formed by: (1) coating a substrate with a first cladding layer (of, e.g., SiO.sub.2); (2) etching a waveguide pattern in the first cladding layer; (3) providing an optical wave-guiding medium (e.g., an organic polymer) in the etched waveguide pattern; and (4) covering the optical wave-guiding medium and adjacent portions of the first cladding layer with a second cladding layer (also of, e.g., SiO.sub.2). Electrode structures may be formed on the cladding layers in patterns determined by the intended function of the waveguide.
In general, techniques used to form thin films from organic electro-optic materials (e.g., spin techniques and dip-coating techniques) do not produce films that are electro-optic in character, because films produced by such techniques generally have a centrosymmetric bulk structure. A molecular alignment process is needed to form a noncentrosymmetric bulk structure in a thin film formed from an organic electro-optic material. An article by K. D. Singer et al., J. Opt. Soc. Am., B4, 968 (1987), discusses an electrically induced poling technique for achieving molecular alignment in a thin-film organic electro-optic material. Furthermore, a technique for forming a thin film of organic electro-optic material does not generally produce a three-dimensional optical waveguide. A patterning technique is needed to produce a three-dimensional waveguide from the thin film of electro-optic material.
It was known in the prior art that when certain non-linear optical polymer materials are "poled" (i.e., when dipolar moieties of the constituent polymers of such materials are aligned), the indices of refraction of the materials are thereby altered. When a modulating electric field is applied to an optical waveguide made of such a poled non-linear optical polymer material, field-dependent changes are thereby superimposed upon the altered index of refraction that is characteristic of the poled polymer material. Thus, by applying a modulating electric field to an optical waveguide made of a poled non-linear optical polymer, phase modulation of optical radiation transmitted through the waveguide can be achieved.
A standard approach to the formation of optical waveguides from poled non-linear optical polymers is the "mask and etch" process, which is discussed by D. L. Lee in Electromagnetic Principles of Integrated Optics, (Wiley, New York, 1986), Chapter 7. Sequential steps in the "mask and etch" process are illustrated in FIGS. 1, 2 and 3.
As shown in FIG. 1, a layer 10 of a non-linear optical polymer is deposited by a conventional technique on a planar substrate 11 made of a chemically stable material such as a glass or fused silica. The substrate 11 with the layer 10 of polymer thereon is then sandwiched between a pair of electrodes 12 and 13 of opposite polarities to produce a poling electric field in the polymer. While the poling field is being maintained in the polymer, the polymer is heated to a temperature above the glass-transition temperature so that dipolar moieties of the polymer are brought to a mobile condition. The dipolar moieties, while in the mobile condition, are urged into alignment by the poling electric field. After a sufficient percentage of the mobile dipolar moieties have become aligned so as to raise the index of refraction of the polymer for a given polarization to a preselected value, the polymer is then cooled below the glass-transition temperature so as to return the dipolar moieties to a stable (i.e., non-mobile) condition while the electric field is still being maintained in the polymer. The electric field is maintained in the polymer for a sufficient time while the polymer is being cooled, so that the dipolar moieties remain permanently aligned after returning to the stable condition.
To form a waveguide (or network of waveguides) from the "poled" non-linear optical polymer material, a mask 14 is formed by a conventional photolithographic technique on the polymer layer 10 as indicated in FIG. 2. The mask 14 is configured to define the shape of the waveguide (or waveguide network). The portion of the polymer layer 10 that is not covered by the mask 14 is then removed from the substrate 11 by a conventional etching technique.
As shown in FIG. 3, only that portion of the polymer layer 10 comprising the waveguide (or waveguide network) remains on the substrate 11. A beam of optical radiation of preselected TM or TE modes generated by a source 15 is guided via the waveguide to a switching region, as determined by the type of device. The optical radiation propagated through the waveguide can be phase-modulated by disposing a pair of electrodes 16 and 17 of opposite polarities so as to impress an electric field on a portion of the waveguide, thereby superimposing field-dependent changes on the index of refraction of the waveguide.
In U.S. Pat. Nos. 3,801,185 (Ramaswamy et al.) and 3,802,760 (Sosnowski et al.), other techniques are disclosed for forming three-dimensional optical waveguides from certain types of thin-film materials for which the index of refraction can be altered by an applied electromagnetic field.